This invention relates to an apparatus for measuring specification ingredients, i.e., oxygen density, in exhaust gas, and for adjusting air fuel ratio or diagnosing an exhaust gas cleaning apparatus such as a catalyst.
Particularly, the present invention relates to the exhaust gas control device for the internal combustion engine, which is suitable for detecting the exhaust gas leaked from an exhaust system, and for preventing an increase or mis-judgement of harmful exhaust gas by the leak.
Oxygen density in exhaust gas is measured and the device for controlling air fuel ratio by feeding the measured exhaust gas back is widely known. In particular, by adjusting the air fuel ratio into a theoretically stoichiometric air fuel ratio and by providing a three way catalyst for the exhaust system, the exhaust gas cleaning apparatus which purifies HC, CO and NOx which are harmful exhaust gases, is generally used as an exhaust gas purification device for cars.
Furthermore, when the three way catalyst and the oxygen density sensor being parts composing the exhaust gas cleaning apparatus deteriorate or are broke down, the harmful exhaust gas is discharged into the atmosphere, and a diagnostic means for checking the deterioration and the trouble of these parts is widely used too. For example, the diagnostic means of the catalyst is disclosed in Japanese Patent Laid-open No. 4-292554 (1992) corresponding to U.S. Pat. No. 5,237,818.
In the devices stated above, a problem to solve in the case that the leak occurred in exhaust system is not considered. When a leak occurs in an exhaust system (for example, a hole opens in exhaust pipe, and the joining point of the parts is moderated), and load of the internal combustion engine is low and turning speed is low, a negative pressure occurs while the exhaust gas pressure is in pulsating. On account of this, air is drawn from the atmospheric air side into the exhaust system.
In this way, the influence in the case that the leak occurred is different according to a position where the leak generates. For example, when the leak occurres between oxygen density sensor for controlling the air fuel ratio and the catalyst, the air fuel ratio is controlled by feeding back so that the exhaust gas becomes theoretically stoichiometric air fuel ratio in a location of the oxygen density sensor, and the oxygen is excessively provided as the air is inhaled in the catalyst location. On account of this, NOx which is a harmful gas is not converted by catalyst, and is discharged into the atmosphere.
Furthermore, as disclosed in Japanese Patent Laid-open 4-292554(1992), when the oxygen density sensor is arranged downstream of the catalyst too and the catalyst is checked by an output signal of the oxygen density sensors positioned up and down stream of the catalyst, the oxygen density sensor down stream of the catalyst always becomes in lean state(oxygen excess), and it become impossible to check the catalyst, thereby a mis-judgement takes place.
When a leak occurres upstream of the oxygen density sensor for the air fuel ratio control, the fuel consumption generally increases because the feedback control is operated so as to obtain the theoretically stoichiometric air fuel ratio at oxygen density sensor location and to increase the fuel amount balancing with the oxygen inhaled by the leak in quantity.
As the Oxygen density sensor generates lean output corresponding to the negative pressure generated by exhaust-gas pressure pulsation. Furthermore, a fault occurs in diagnosing the oxygen sensor, the air-fuel ratio becomes more excessive than usual, so the harmful gas is discharged into the atmosphere as a result.
Furthermore, in the case that the leak is generated up stream of the catalyst, the exhaust-gas which isn't purified is discharged into the atmosphere when the exhaust-gas pressure is positive.